Devices for measurements of the aforementioned type have been marketed by several companies and, for example, such devices include magnetically inductive flow meters (MIF) as well as rotary vane meters, the rotational speed of which are scanned by various means, for example, conductive measurement, the eddy current principle, and an ultrasonic reflexing indexing method.
Additionally, systems using anemometer principles as well as the heat pulse method are known and are used in practice. Furthermore, static ultrasonic methods are used in which ultrasonic transmitters and receivers located in the flow tube by various method as, for example, phase measurement, travel time measurement, frequency measurement and tracking, for measuring flow have been proposed.
The systems of the prior art suffer from the definite disadvantages including excessive energy consumption, excessive production costs, sensitivity to contamination, mechanically moving parts, and insufficiently wide dynamic range, problems with "creep" flows and problems caused by external factors, such as solid particles and air bubbles.
Although known ultrasonic methods solve some of the above-described problems, the ultrasonic methods are characterized by sever demands on the accuracy of mechanical manufacture, high losses caused multi-reflection, time-intensive searching and regulating procedures, and high energy consumption. For these reasons, these systems cannot be produced with a sufficiently high time resolution at low manufacturing prices.